The present invention relates to an electrolyte, an ion conductor including the electrolyte, and electrochemical devices including the electrolyte, such as lithium cells, lithium ion cells, electrical double-layer capacitors.
Accompanying the evolution of portable equipment in recent years, there has been active development of electrochemical devices utilizing electrochemical phenomena, such as cells for use as their power supplies and capacitors. In addition, electrochromic devices (ECD), in which a color change occurs due to an electrochemical reaction, are examples of electrochemical devices for uses other than power supplies.
These electrochemical devices are typically composed of a pair of electrodes and an ion conductor filled between them. The ion conductor contains a salt (AB) as an electrolyte, which is dissolved in a solvent, polymer or mixture thereof such that the salt is dissociated into cations (A+) and anions (Bxe2x88x92), resulting in ionic conduction. In order to obtain the required level of ion conductivity for the device, it is necessary to dissolve a sufficient amount of this electrolyte in solvent or polymer. In actuality, there are many cases in which a solvent other than water is used, such as organic solvents and polymers. Electrolytes having sufficient solubility in such organic solvents and polymers are presently limited to only a few types. For example, electrolytes having sufficient solubility for use in lithium cells are only LiClO4, LiPF6, LiBF4, LiAsF6, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9) and LiCF3SO3. Although the cation type of the electrolyte is frequently limited by the device as is the case with the lithium ion of lithium cells, any anion can be used for the electrolyte provided it satisfies the condition of having high solubility.
Amidst the considerable diversity of the application range of these devices, efforts are made to seek out the optimum electrolyte for each application. Under the present circumstances, however, optimization efforts have reached their limit due to the limited types of available anions. In addition, existing electrolytes have various problems, thereby creating the need for an electrolyte having a novel anion portion. More specifically, since ClO4 ion of LiClO4 is explosive and AsF6 ion of LiAsF6 is toxic, they cannot be used for reasons of safety. Even the only practical electrolyte of LiPF6 has problems including heat resistance and hydrolysis resistance. Although electrolytes of LiN(CF3SO2)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9) and LiCF3SO3 are stable and high in ionic conductivity, they corrode the aluminum collector inside the cell when a potential is applied. Therefore, their use presents difficulties.
It is therefore an object of the present invention to provide a useful novel electrolyte, a novel ion conductor containing the electrolyte, and a novel electrochemical device containing the ion conductor.
According to the present invention, there is provided an electrolyte for an electrochemical device. This electrolyte comprises:
a first compound that is an ionic metal complex represented by the general formula (1); and
at least one compound selected from the group consisting of second to fourth compounds respectively represented by the general formulas (2) to (4), fifth to ninth compounds respectively represented by the general formulas Aa+(PF6xe2x88x92)a, Aa+(ClO4xe2x88x92)a, Aa+(BF4xe2x88x92)a, Aa+(AsF6xe2x88x92)a, and Aa+(SbF6xe2x88x92)a, and tenth to twelfth compounds respectively represented by the general formulas (5) to (7), 
wherein M is a transition metal selected from the group consisting of elements of groups 3-11 of the periodic table, or an element selected from the group consisting of elements of groups 12-15 of the periodic table;
Aa+ represents a metal ion, onium ion or hydrogen ion;
a represents a number from 1 to 3; b represents a number from 1 to 3; p is b/a; m represents a number from 1 to 4; n represents a number from 0 to 8; q is 0 or 1;
X1 represents O, S, NR5 or NR5R6;
each of R1 and R2 independently represents H, a halogen, a C1-C10 alkyl group or C1-C10 halogenated alkyl group;
R3 represents a C1-C10 alkylene group, C1-C10 halogenated alkylene group, C4-C20 arylene group or C4-C20 halogenated arylene group, these alkylene and arylene groups of said R3 optionally having substituents and hetero atoms, one of said R3 being optionally bonded with another of said R3;
R4 represents a halogen, C1-C10 alkyl group, C1-C10 halogenated alkyl group, C4-C20 aryl group, C4-C20 halogenated aryl group or X2R7, these alkyl and aryl groups of said R4 optionally having substituents and hetero atoms, one of said R4 being optionally bonded with another of said R4;
X2 represents O, S, NR5 or NR5R6;
each of R5 and R6 represents H or a C1-C10 alkyl group;
R7 represents a C1-C10 alkyl group, C1-C10 halogenated alkyl group, C4-C20 aryl group or C4-C20 halogenated aryl group;
each of x, y and z independently represents a number from 1 to 8
each of Y1, Y2 and Y3 independently represents a SO2 group or CO group; and
each of R8, R9 and R10 independently represents an electron-attractive organic substituent optionally having a substituent or a hetero atom, at least two of said R8, R9 and R10 being optionally bonded together to form a ring, at least one of said R8, R9 and R10 being optionally bonded with an adjacent molecule to form a polymer.
According to the present invention, there is provided an ion conductor for an electrochemical device. This ion conductor comprises the electrolyte; and a member selected from the group consisting of a nonaqueous solvent, a polymer and a mixture thereof, said member dissolving therein said electrolyte.
According to the present invention, there is provided an electrochemical device comprising (a) first and second electrodes; and (b) the ion conductor receiving therein said first and second electrodes.
An electrolyte according to the present invention is superior in cycle characteristics and shelf life as compared with conventional electrolytes. Thus, the electrolyte can advantageously be used for electrochemical devices such as lithium cell, lithium ion cell and electrical double-layer capacitor.
According to the invention, the alkyl groups, halogenated alkyl groups, aryl groups and halogenated aryl groups, which are contained in the ionic metal complex and the raw materials for synthesizing the same, may be branched and/or may have other functional groups such as hydroxyl groups and ether bonds.
The followings are specific nine examples of the ionic metal complex represented by the general formula (1) of the present invention. 
Here, although lithium ion is indicated as an example of Aa+ of the general formula (1), examples of other cations that can be used other than lithium ion include sodium ion, potassium ion, magnesium ion, calcium ion, barium ion, cesium ion, silver ion, zinc ion, copper ion, cobalt ion, iron ion, nickel ion, manganese ion, titanium ion, lead ion, chromium ion, vanadium ion, ruthenium ion, yttrium ion, lanthanoid ion, actinoid ion, tetrabutylammonium ion, tetraethylammonium ion, tetramethylammonium ion, triethylmethylammonium ion, triethylammonium ion, pyridinium ion, imidazolium ion, hydrogen ion, tetraethylphosphonium ion, tetramethylphosphonium ion, tetraphenylphosphonium ion, triphenylsulfonium ion, and triethylsulfonium ion. In the case of considering the application of the ionic metal complex for electrochemical devices and the like, lithium ion, tetraalkylammonium ion and hydrogen ion are preferable. As shown in the general formula (1), the valency (valence) of the Aa+ cation is preferably from 1 to 3. If the valency is larger than 3, the problem occurs in which it becomes difficult to dissolve the ionic metal complex in solvent due to the increase in crystal lattice energy. Consequently, in the case of requiring solubility of the ionic metal complex, a valency of 1 is preferable. As shown in the general formula (1), the valency (bxe2x88x92) of the anion is similarly preferably from 1 to 3, and a valency of 1 is particularly preferable. The constant p expresses the ratio of the valency of the anion to the valency of the cation, namely b/a.
In the general formula (1), M at the center of the ionic metal complex of the present invention is selected from elements of groups 3-15 of the periodic table. It is preferably Al, B, V, Ti, Si, Zr, Ge, Sn, Cu, Y, Zn, Ga, Nb, Ta, Bi, P, As, Sc, Hf or Sb, and more preferably Al, B or P. Although it is possible to use various elements for the M other than these preferable examples, synthesis is relatively easy in the case of using Al, B, V, Ti, Si, Zr, Ge, Sn, Cu, Y, Zn, Ga, Nb, Ta, Bi, P, As, Sc, Hf or Sb. In addition to ease of synthesis, the ionic metal complex has excellent properties in terms of low toxicity, stability and production cost in the case of using Al, B or P.
In the general formula (1), the organic or inorganic portion bonded to M is referred to as the ligand. As mentioned above, X1 in the general formula (1) represents O, S, NR5 or NR5R6, and is bonded to M through its hetero atom (O, S or N). Although the bonding of an atom other than O, S or N is not impossible, the synthesis becomes extremely bothersome. The ionic metal complex represented by the general formula (1) is characterized by these ligands forming a chelate structure with M since there is bonding with M by a carboxyl group (xe2x80x94COOxe2x88x92) other than X1 within the same ligand. As a result of this chlelation, the heat resistance, chemical stability and hydrolysis resistance of the ionic metal complex are improved. Although constant q in this ligand is either 0 or 1, in the case of 0 in particular, since the chelate ring becomes a five-member ring, chelating effects are demonstrated most prominently, making this preferable due to the resulting increase in stability. In addition, since the negative charge of the central M is dissipated by electron attracting effects of the carboxyl group(s) resulting in an increase in electrical stability of the anion, ion dissociation becomes extremely easy resulting in corresponding increases of the ionic metal complex in solvent solubility, ion conductivity, catalyst activity and so forth. In addition, the other properties of heat resistance, chemical stability and hydrolysis resistance are also improved.
In the general formula (1), each of R1 and R2 is independently selected from H, halogen, C1-C10 alkyl groups and C1-C10 halogenated alkyl groups. At least one of either R1 and R2 is preferably a fluorinated alkyl group, and more preferably, at least one of R1 and R2 is a trifluoromethyl group. Due to the presence of an electron-attracting halogen and/or a halogenated alkyl group for R1 and R2, the negative charge of the central M is dissipated. This results in an increase of the anion of the general formula (1) in electrical stability. With this, the ion dissociation becomes extremely easy resulting in an increase of the ionic metal complex in solvent solubility, ion conductivity, catalyst activity and so forth. In addition, other properties of heat resistance, chemical stability and hydrolysis resistance are also improved. The case in which the halogen is fluorine in particular has significant advantageous effects, while the case of a trifluoromethyl group has the greatest advantageous effect.
In the general formula (1), R3 is selected from C1-C10 alkylene groups, C1-C10 halogenated alkylene groups, C4-C20 arylene groups and C4-C20 halogenated arylene groups. R3 is preferably one which forms a 5 to 10-membered ring when a chelate ring is formed with the central M. The case of a ring having more than 10 members is not preferable, since chelating advantageous effects are reduced. In addition, in the case R3 has a portion of hydroxyl group or carboxyl group, it is possible to form a bond between the central M and this portion.
In the general formula (1), R4 is selected from halogens, C1-C10 alkyl groups, C1-C10 halogenated alkyl groups, C4-C20 aryl groups, C4-C20 halogenated aryl groups and X2R7. Of these, fluorine is preferable. X2 represents O, S, NR5 or NR5R6 and bonds to M through one of these heteroatoms (O, S and N). Although the bonding of an atom other than O, S or N is not impossible, the synthesis becomes extremely bothersome. Each of R5 and R6 is selected from H and C1-C10 alkyl groups. Each of R5 and R6 differs from other groups (e.g., R1 and R2) in that the former is not required to be an electron attracting group. In the case of introducing an electron attracting group as R5 or R6, the electron density on N of NR5R6 decreases, thereby preventing coordination on the central M. R7 is selected from C1-C10 alkyl groups, C1-C10 halogenated alkyl groups, C4-C20 aryl groups and C4-C20 halogenated aryl groups. Of these, a C1-C10 fluorinated alkyl groups is preferable. Due to the presence of an electron-attracting halogenated alkyl group as R7, the negative charge of the central M is dissipated. Since this increases the electrical stability of the anion of the general formula (1), ion dissociation becomes extremely easy resulting in an increase of the ionic metal complex in solvent solubility, ion conductivity and catalyst activity. In addition, other properties of heat resistance, chemical stability and hydrolysis resistance are also improved. The case in which the halogenated alkyl group as R7 is a fluorinated alkyl group in particular results in even greater advantageous effects.
In the general formula (1), the values of the constants m and n relating to the number of the above-mentioned ligands depend on the type of the central M. In fact, m is preferably from 1 to 4, while n is preferably from 0 to 8. Furthermore, m may be from 1 to 3, while n may be from 0 to 4.
According to a first preferred embodiment of the invention, the electrolyte contains the ionic metal complex represented by the general formula (1) and another component that is at least one compound selected from the above-mentioned second to fourth compounds represented by the general formulas (2), (3) and (4). Examples of these compounds are LiCF3SO3, LiN(CF3SO2)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9) and LiC(CF3SO2)3. If the ionic metal complex is omitted in the first preferred embodiment, the following problem occurs. That is, the another component corrodes the aluminum collector inside the cell when a potential is applied. With this, the capacity is lowered by repeating the charge and discharge cycle. In contrast, according to the first preferred embodiment, the aluminum collector corrosion can unexpectedly be prevented by using a mixture of the ionic metal complex and the another component. The reason of this is not clear. It is, however, assumed that the ionic metal complex is slightly decomposed on the electrode surface and that a film of the ionic metal complex""s ligand is formed on the aluminum surface, thereby preventing the aluminum collector corrosion.
According to a second preferred embodiment of the invention, the electrolyte contains the ionic metal complex represented by the general formula (1) and another component that is at least one compound selected from the above-mentioned fifth to ninth compounds respectively represented by the general formulas Aa+(PF6xe2x88x92)a, Aa+(ClO4xe2x88x92)a, Aa+(BF4xe2x88x92)a, Aa+(AsF6xe2x88x92)a, and Aa+(SbF6xe2x88x92)a where Aa+ is preferably the same ion as that in the general formula (1). If the ionic metal complex is omitted in the second preferred embodiment, the following problem occurs. That is, the anion(s) tends to be pyrolyzed at a high temperature of 60xc2x0 C. or higher, thereby generating a Lewis acid(s). This Lewis acid decomposes the solvent and makes the electrochemical device inferior in performance and lifetime. Furthermore, the omission of the ionic metal complex causes hydrolysis of the anion(s) when the electrolyte is contaminated with a very small amount of water. This hydrolysis generates an acid(s) that makes the electrochemical device inferior in performance and lifetime. In contrast, according to the second preferred embodiment, the above-mentioned pyrolysis and hydrolysis can unexpectedly be prevented by using a mixture of the ionic metal complex and the another component. The reason of this is not clear. It is, however, assumed that the properties of the solution as a whole change somehow to achieve this prevention by a certain interaction between the ionic metal complex and the another component.
According to a third preferred embodiment of the invention, the electrolyte contains the ionic metal complex represented by the general formula (1) and another component that is at least one compound selected from the above-mentioned tenth to twelfth compounds represented by the general formulas (5), (6) and (7). Examples of these compounds are CF3CH2OSO3Li, (CF3)2CHOSO3Li, (CF3CH2OSO2)2NLi, ((CF3)2CHOSO2)2NLi, (CF3CH2OSO2)((CF3)2CHOSO2)NLi, ((CF3)2COSO2)2NLi, and ((CF3)2CHOSO2)3CLi. Further examples are polymers and oligomers such as [N(Li)SO2OCH2(CF2)4CH2OSO2]n where n is a number of 2-1,000. If the ionic metal complex is omitted in the third preferred embodiment, the following problem occurs. That is, the another component corrodes the aluminum collector inside the cell when a potential is applied. With this, the capacity is lowered by repeating the charge and discharge cycle. In contrast, according to the third preferred embodiment, the aluminum collector corrosion can unexpectedly be prevented by using a mixture of the ionic metal complex and the another component. The reason of this is not clear. It is, however, assumed that the ionic metal complex is slightly decomposed on the electrode surface and that a film of the ionic metal complex""s ligand is formed on the aluminum surface, thereby preventing the aluminum collector corrosion.
In the invention, the molar ratio of the ionic metal complex to the at least one compound is preferably 1:99 to 99:1 (or a range from 5:95 to 95:5), more preferably 20:80 to 80:20 (or a range from 30:70 to 70:30), in view of improving the electrochemical device in cycle characteristics and shelf life. If this ratio is less than 1:99 (or 5:95), it may become insufficient to prevent the above-mentioned aluminum corrosion and/or the above-mentioned pyrolysis and hydrolysis, thereby making the electrolyte inferior in cycle characteristics and shelf life. If the ratio is greater than 99:1 (or 95:5), advantages of adding the at least one compound to increase ionic conductivity and electrochemical stability may become insufficient.
There are no particular restrictions on the process for synthesizing the ionic metal complex. An exemplary process for producing the ionic metal complex is disclosed in European Patent Application EP 1075036 A2.
In the case of preparing an electrochemical device of the present invention, its basic structural elements are ion conductor, negative electrode, positive electrode, collector, separator, container and the like.
A mixture of electrolyte and non-aqueous solvent or polymer is used as the ion conductor. If a non-aqueous solvent is used, the resulting ion conductor is typically referred to as an electrolytic solution, while if a polymer is used, it is typically referred to as a polymer solid electrolyte. Non-aqueous solvent may also be contained as plasticizer in polymer solid electrolytes.
There are no particular restrictions on the non-aqueous solvent provided it is an aprotic solvent that is able to dissolve an electrolyte of the present invention, and examples of this non-aqueous solvent that can be used include carbonates, esters, ethers, lactones, nitrites, amides and sulfones. In addition, the solvent can either be used alone or in the form of a mixture of two or more types of solvent. Specific examples of the solvent include propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, methylethyl carbonate, dimethoxyethane, acetonitrile, propionitrile, tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, nitromethane, N, N-dimethylformamide, dimethylsulfoxide, sulfolane and xcex3-butyrolactone.
In case that Aa+ of the general formula (1) is lithium ion, the non-aqueous solvent of an electrolytic solution is preferably a mixture of a first aprotic solvent having a dielectric constant of 20 or greater and a second aprotic solvent having a dielectric constant of 10 or less. In fact, lithium salt has a low solubility in the second aprotic solvent (e.g., diethyl ether and dimethyl carbonate). Therefore, it may be difficult to obtain a sufficient ionic conductivity by using only the second aprotic solvent. In contrast, lithium salt has a high solubility in the first aprotic solvent. The resulting solution is, however, high in viscosity. Thus, it may be difficult to obtain a sufficient ionic conductivity by using only the first aprotic solvent, too. In contrast, it becomes possible to gain a suitable solubility and a suitable ionic mobility by using a mixture of the first and second aprotic solvents, thereby making it possible to obtain a sufficient ionic conductivity.
There are no particular restrictions on the polymer to be mixed with the electrolytes of the invention provided it is an aprotic polymer. Examples of such polymer include polymers having polyethylene oxide on their main chain or side chain, homopolymers or copolymers of polyvinylidene fluoride, methacrylate polymers and polyacrylonitrile. In the case of adding plasticizer to these polymers, the above-mentioned aprotic non-aqueous solvent can be used. The total concentration of the electrolytes of the present invention in these ion conductors is preferably 0.1 mol/dm3 or more up to the saturated concentration, and more preferably from 0.5 mol/dm3 to 1.5 mol/dm3. If the concentration is lower than 0.1 mol/dm3, ion conductivity may become too low.
There are no particular restrictions on the negative electrode material for preparing an electrochemical device. In the case of lithium cell, lithium metal (metallic lithium) or an alloy of lithium and another metal can be used. In the case of a lithium ion cell, it is possible to use an intercalation compound containing lithium atoms in a matrix of another material, such as carbon, natural graphite or metal oxide. This carbon can be obtained by baking polymer, organic substance, pitch or the like. In the case of electrical double-layer capacitor, it is possible to use activated carbon, porous metal oxide, porous metal, conductive polymer and so forth.
There are no particular restrictions on the positive electrode material. In the case of lithium cell or lithium ion cell, lithium-containing oxides such as LiCoO2, LiNiO2, LiMnO2 and LiMn2O4; oxides such as TiO2, V2O5 and MoO3; sulfides such as TiS2 and FeS; and electrically conductive polymers such as polyacetylene, polyparaphenylene, polyaniline or polypyrrole can be used. In the case of electrical double-layer capacitor, activated carbon, porous metal oxide, porous metal, electrically conductive polymer and so forth can be used.
The following nonlimitative examples are illustrative of the present invention. In fact, Examples 1xe2x80x941 to 1-5 are illustrative of the above-mentioned first preferred embodiment of the invention, and Examples 2-1 to 2-4 are illustrative of the above-mentioned second preferred embodiment of the invention, and Examples 3-1 to 3-4 are illustrative of the above-mentioned third preferred embodiment of the invention.